In process automation technology, as well as manufacturing automation technology, field devices are often employed, which serve to register and/or influence process variables. For the registering of process variables, measuring devices are used, which, in each case, exhibit at least one sensor and one measurement transmitter. Such measuring devices include, for example, fill-level measuring devices, flow measuring devices, pressure and temperature measuring devices, pH-redox potential measuring devices, electrical conductivity measuring devices, etc., which ascertain the respective process variables of fill-level, flow, pressure, temperature, pH-value and conductivity. For influencing process variables, actuators are used, for example valves or pumps, via which the flow of a fluid in a section of pipeline or the fill-level in a container can be changed.
In principle, all devices which are employed near the process and which deliver or work with process-relevant information are referred to as field devices. In addition to the aforementioned measuring devices/sensors and actuators, such units that are directly connected to a fieldbus and which serve to communicate with superordinated units (e.g. remote I/Os, gateways, linking devices and wireless adapters) are also generally referred to as field devices. A large number of these devices are produced and sold by the Endress+Hauser Group.
In modern industrial facilities, field devices are, as a rule, connected with superordinated units via fieldbus systems (e.g. Profibus®, Foundation Fieldbus®, HART®, etc.). Normally, the superordinated units involve control systems or control units, for example a PLC (programmable logic controller). The superordinated units are used, for example, in process controlling, process visualizing, process monitoring as well as in the start-up of the field devices. The measured values registered by the field devices—especially by the sensors—are transmitted via the connected bus system to a superordinated unit, or, as the case may be, to several superordinated units. Additionally, a transfer of data from the superordinated unit to the field devices via the bus system is necessary; this is used especially in the configuring and parametering of field devices or for diagnostic purposes. Generally speaking, the field device is serviced from the superordinated unit via the bus system.
In addition to a hardwired data transmission between the field devices and the superordinated unit, the possibility of a wireless data transmission also exists. In particular, in the case of the bus systems Profibus®, Foundation Fieldbus® and HART®, a wireless data transmission via radio is specified. Additionally, radio networks for sensors are more precisely specified in the standard IEEE 802.15.4.
For the realization of a wireless transmission of data, field devices are embodied, for example, as radio-field devices. As a rule, these exhibit a radio unit and an electrical current source as integral components. In such a case, the radio unit and the electrical current source can be provided in the field device itself, or in a radio module which is permanently connected to the field device. Through the electrical current device, an autarkic energy supply for the field device is made possible.
Alternatively, field devices without radio units—i.e. the current installed base in the field—are upgraded to a radio-field device through the attachment of a wireless adapter which features a radio unit. A corresponding wireless adapter is described, for example, in the publication, WO 2005/103851 A1. The wireless adapter is, as a rule, connected to a fieldbus communication interface of the field device in a detachable manner. Via the fieldbus communication interface, the field device can transmit data over the bus system to the wireless adapter, which then transmits this via radio to the target location. Conversely, the wireless adapter can receive data via radio and forward it over the fieldbus communication interface to the field device. The supplying of the field device with electrical power occurs then, as a rule, via an energy supply unit associated with the wireless adapter.
The greater part of the installed base of field devices is made up of HART-devices, which communicate with a superordinated control unit via the HART standard. In order to integrate 4-20 mA/HART into wireless networks, adapters (as previously mentioned) are employed which implement the communication link to the wireless network and, in such case, also cyclically-automatedly register measured values from the connected field device. Suitable adapters have at their disposal an autarkic energy source (e.g. batteries), which supplies both the adapter as well as the connected field device with energy. For powering the connected field device, the adapter exhibits a power supply stage, which supplies the required terminal voltage at the appropriate 4-20 mA measuring and supply current. Due to the relatively high energy consumption of today's 4-20 mA field devices, such devices are normally not continually supplied with energy, but only according to need. This clocked operation of the field device is also known as duty cycle operation.
Field devices require, at a defined measurement, or supply, current, a defined minimum terminal voltage for regular operation. The measurement, or supply, current changes dynamically during operation. The different operating phases are listed below:                Operating phase, “measurement with 4-20 mA output”: Here, the electrical current changes proportional to the measured value.        Operating phase, “measurement with HART only output”: Here, the field device is operated in multidrop mode. During operation, the electrical current is consistently set to a minimum value.        Operating phase, “start-up”: Here, a deviating electrical current is present up to the ascertainment of the first measured value.        
A corresponding I/O module of a wireless adapter has in the measuring loop internally various resistors, which, at constant internal supply voltage, provide for the connected measurement transmitter a terminal voltage dependent on electrical current flow. These resistors are, for example, an Ex limiting resistor, a HART communication resistor or a measuring resistor for registering the analog 4-20 mA measurement current. In order to compensate for the voltage drops caused by the internal series-connected resistors, the internal supply voltage of the power supply must be chosen in such a way that the defined terminal voltage of the field device is reached even in the case of the highest arising electrical current. The maximum arising current is the error current, which amounts to, for example, 22 mA. The listed resistors, with the exception of the Ex limiting resistor, are, furthermore, not absolutely required in every operating phase; they currently must, however, always be taken into consideration when setting the supply voltage. Due to this internal supply voltage being rigidly set for the worst-case-scenario, the available energy is not utilized efficiently. In many operating phases, a not insignificant portion of the available energy is wasted.